Recently, energy saving has been actively promoted to protect the environment. For battery-powered portable equipment, such as mobile phones, digital cameras, and the like, energy efficiency is especially important to prolong battery life. Such portable equipment typically uses a switching regulator that includes an inductor because it is efficient and can be made compact.
FIG. 1 is a circuit diagram of a known switching regulator. The switching regulator 100 is a step-down synchronous-rectification type switching regulator. In FIG. 1, the switching regulator 100 includes a switching transistor S101, a synchronous rectification transistor S102, an inductor L101, an output capacitor C101, comparators 103, 105, and 107, an oscillator 104, an output driver 106, and bleeder resistors R101 and R102. A load 110 is connected between an output terminal OUT (Vout) and ground.
The switching regulator 100 operates in a continuity mode and a discontinuity mode. In the continuity mode, a current flows through the inductor L101 continuously. By contrast, in the discontinuity mode, the current does not flow through the inductor L 101 continuously. Consequently, when the switching regulator operates in the discontinuity mode under light load conditions, a reverse current may flow from the output,terminal OUT to ground through the synchronous rectification transistor S102.
To avoid such reverse current, the switching regulator 100 employs the comparator 107 which compares a voltage VLx at a junction node Lx between the switching transistor S101 and the synchronous rectification transistor S102 with a ground potential GND. When the voltage VLx at the junction node Lx exceeds the ground potential GND, the synchronous rectification transistor S102 is cut off so that the reverse current is avoided.
However, in the switching regulator 100 shown in FIG. 1, the synchronous rectification transistor S102 may not be cut off before generation of the reverse current due to a delay at the comparator 107, resulting in the reverse current flowing from the output terminal OUT to the inductor L101. As a result, performance efficiency of the switching regulator decreases.
The ground potential GND input to inverted input terminal of the comparator 107 may be replaced by a low voltage that is lower than the ground potential GND by a predetermined voltage. Then, the comparator 107 compares the voltage VLx at the junction node Lx with the low voltage so as to detect an indication of the reverse current. Accordingly, it is possible to cut off the synchronous rectification transistor S102 before generation of the reverse current, thus avoiding the reverse current. In this switching regulator 100, however, a voltage change of the voltage VLx at the junction node Lx is relatively small with respect to time. Accordingly, it takes a long time to obtain enough of a voltage change to cause the comparator 107 to invert the output voltage thereof. To cause the comparator 107 to invert the output voltage with such small voltage change, it is necessary to increase a supply current to the comparator 107. When the supply current to the comparator 107 is increased, however, the performance efficiency of the switching regulator 100 decreases due to large power consumption under light load conditions.